The present invention pertains to a method and apparatus for processing a video signal. More particularly, the present invention pertains to such a method and process for removing modulation from the color burst component of a video signal.
In a copending application entitled Method and Apparatus For Processing a Video Signal, Ser. No. 551,696 now U.S. Pat. No. 4,577,216, filed Nov. 14, 1983, there is disclosed a method and apparatus for processing a color video signal in a manner such as to inhibit the making of acceptable video tape recordings thereof. However, the processing of the color video signal is such that a conventional television receiver produces a normal color picture from the modified signal.
In both the NTSC and PAL color television systems color information is transmitted on a subcarrier signal. In both systems the precise color at any point in the picture is determined by the phase of the subcarrier relative to some reference phase, while the degree of saturation of the color is determined by the amplitude of the subcarrier. In order for a television receiver to correctly reproduce colors, the receiver requires information concerning the above-mentioned reference phase. This information is transmitted as part of the video signal in the form of a burst of about 9 cycles of subcarrier following the horizontal synchronizing pulse, and is referred to as a color burst. This color burst signal is used in the television receiver to phase lock a crystal oscillator, thus generating a continuous subcarrier signal at the reference phase which is then used to demodulate the color information. It is normal for the phase-locked crystal oscillator in the television receiver to have a fairly long time constant, on the order of a few milliseconds. The long time constant ensures that the oscillator will ignore short term phase perturbations of the color burst signal as might be caused by noise.
In accordance with applicant's copending application Ser. No. 551,696, the color burst signal of a color video signal is phase modulated with a noise signal to produce a modified signal. The modified signal is such that the phase modulation is ignored by a television receiver, which produces a normal color picture from the color video signal.
As far as color video tape recorders are concerned, it is common practice to utilize the color burst component of a video signal to generate velocity error-correction signals to compensate for fluctuations of the head to tape velocity. The velocity error corrector circuit in a video tape recorder measures the phase of the color burst signal on a line-by-line basis. Any variation of phase is assumed to be due to head to tape velocity errors, and a correction signal is generated. The correction signal alters the phase of the chrominance signal in an attempt to eliminate these errors.
Since in accordance with the invention disclosed in applicant's copennding application Ser. No. 551,696, deliberate phase errors are introduced into the color burst signal, the velocity error correction circuit in the video tape recorder will attempt to eliminate the deliberate phase errors thus introduced. In so doing, the velocity error correction circuit will transfer the error from the color burst to the chrominance signal, thus giving rise to severe color noise in the viewed picture, and inhibiting the making of acceptable video tape recordings of the color video signal.
As disclosed in applicant's copending application, various types of phase modulation are suitable. The phase modulation can be any suitable multivalued noise signal for driving a phase modulator. Alternatively, the color burst signal can be caused to randomly assume one of two phase angles equally displaced about its nominal value. In fact, random, pseudo-random, periodic signals and combinations thereof can be used for the phase modulation. Since the filling of applicant's copening application Ser. No. 551,696 it has been found that a best mode embodiment of the invention there disclosed and claimed may be alternate phase reversals of the color burst signal, i.e. modulating the phase by 180.degree..